anchev wrote:

Negative RGB values? Why?

.

Why not?

Pretending middle gray is zero is a big pain that is unnecessary in floating point.

j francis photography wrote:
Why not?

Because there is no such thing as negative color, especially in additive color model.

Pretending middle gray is zero is a big pain that is unnecessary in floating point.

But 16 bit color is integer, not FP.

j francis photography wrote:
What does the subtraction blend mode even mean if negative numbers are disallowed?

https://en.wikipedia.org/wiki/Blend_modes#Subtract

"This blend mode simply subtracts pixel values of one layer with the other. In case of negative values, black is displayed."

In other words (pseudo code):

a = (b - c) > 0 ? (b-c) : 0

// No negative values needed.

How do you sum noise of various frequencies and amplitudes if all the noise is positive? Use overlay and pretend it's summing?

Are you suggesting that camera sensors have an extra bit too just to store additional noise?

j francis photography wrote:
This topic seems to have come up quite a bit over the years. Here's a quote from someone in 2010 that sounds interesting:

Photoshop's "15 bit" internal values are actually 15 bits (0..32767) plus 1 (so 32768). That scheme is apparently chosen to allow faster image data math while avoiding calculation over- or underflow, and support other undisclosed (blending) tricks.


http://www.openphotographyforums.com/fo … hp?t=12522

Thanks for the link. It seems to confirm my findings about bit depth. However it doesn't justify the lack of accuracy in the other tools. The whole thing sounds like someone did a really dirty code writing.

anchev wrote:
That's like saying that retouching is something which doesn't require accurate color

FWIW: Actually I noticed it visually while testing some gradient stuff. I was able to see banding on a gradient and that made me question if it is really a 16-bit gradient and then I made all the testing.

Banding on a gradient?

Aren't gradients dithered by Photoshop?

j francis photography wrote:
Banding on a gradient?

Aren't gradients dithered by Photoshop?

It's an option. But there is no such option for gradients which you create in masks.

j francis photography wrote:
How do you sum them in Photoshop to make this?

If you have negative values and the images go from -amplitude to +amplitude centered afround zero you just sum them. Then you normalize them at display time to make them displayable.

How do you do that in Photoshop?

You should ask the developers of PS. I didn't write this program, so I can't tell you what formula exactly it uses.

I prefer programs that don't tell me what to do with my numbers.

Me too. And PS doesn't seem to respect the instructions one gives to it. Hence the thread.

But when it comes to creative image editing I'm not going to sweat ONLY having 15 bits when I mean to have 16.

There is nothing creative in image editing. That's why it is called editing, not creating. But whatever words you use, it doesn't justify the lack of promised accuracy of the tools. It's really a purely technical matter. Also in the case of a gradient it is not 16 vs 15 but rather 16 vs 12 as explained.

Mark Salo wrote:
Well, slap me down because I'm probably wrong.  But isn't the first bit (in human language) the zero bit in computer language.  So the 15th bit in computer language is the 16th bit in human language.

You seem to be mixing indexing with bit depth. Yes, usually in computer languages the zero element is the first element. But in 16 bits you still have 2^16 total elements. It is just that the values are indexed as 0-65535 and not 1-65536.

2^0 = 1 - which means with 0 bits you can have only 1 value
2^1 = 2 - which means with 1 bit you can have 2 values (0 or 1)
2^2 = 4 - which means with 2 bits you can have 4 values binary (00, 01, 10, 11) or in decimal (0, 1, 2, 3)
...
2^15 = 32768 (0-32767)
2^16 = 65536 (0-65535)

LightDreams wrote:
Looks like between you and the article you referenced, that it's a pretty solid case that it's actually (just slightly over) 15 bits and not a "full" 16 bits.   I.E. They can truthfully claim to be USING all 16 bits, but the catch is that the 16th bit value is only a numeric plus 1 to make it easier to determine a mid point or use divisional math, etc.

For those that haven't read the linked article, the range of values is exactly the 15 bit total PLUS (the number) ONE, which is the 16th bit, instead of fully using (almost doubling) the additionally available combination values that full use of a 16th bit could provide.

I used to be pissed, in the early 90s, that Photoshop wasn't floating point like all the proprietary software I was accustomed to.

anchev wrote:
In which case it would be justified users to pay for only 6 months + 1 day of subscription and this should be considered full 12 months

Your logic, as usual, is impeccable. Something tells me ain't gonna happen, though.

FKW wrote:
We dont see the number

But we see the result of it and if the number is wrong - the result cannot be right. I actually noticed some visual discrepancies related to gradients and then I came to these tests.

Here is something more which demonstrates it:

1) A black to white (supposedly and almost) linear gradient generated in Photoshop (zoomed a lot). Notice the difference in the R channel (personally I can see visually the slight tint variations):



2) The same gradient generated in GIMP - really linear as explained earlier and R=G=B

anchev wrote:
But we see the result of it and if the number is wrong - the result cannot be right. I actually noticed some visual discrepancies related to gradients and then I came to these tests.

Here is something more which demonstrates it:

1) A black to white (supposedly and almost) linear gradient generated in Photoshop (zoomed a lot). Notice the difference in the R channel (personally I can see visually the slight tint variations):



2) The same gradient generated in GIMP - really linear as explained earlier and R=G=B

This is inaccurate because you are in wrong. If you need to create a proper grayscale gradient and then convert it to Photoshop, you must create it correctly from the beginning, otherwise the conversion will be incorrect. How do you do a gradient RGB in Photoshop? You can see my attached are right and linear.

fireshoot wrote:
This is inaccurate because you are in wrong.

There are the steps to reproduce. If you follow them you will see the facts for yourself. I don't quite see how someone could be wrong for sharing actual, verifiable and observable facts.

If you need to create a proper grayscale gradient and then convert it to Photoshop, you must create it correctly from the beginning, otherwise the conversion will be incorrect.

This also makes no sense. It seems you imply that a gradient should be created beforehand outside of Photoshop (which renders Photoshop useless for the job) and it is totally unclear what "convert it to Photoshop" means. What are you talking about?

How do you do a gradient RGB in Photoshop?

Gradient adjustment layer.

You can see my attached are right and linear.

What are you trying to demonstrate with this? It does not reveal steps to reproduce. Also how is it "right and linear" considering your samples are not even evenly spaced? (and are 8-bit)

What are you trying to demonstrate with this? It does not reveal steps to reproduce. Also how is it "right and linear" considering your samples are not even evenly spaced? (and are 8-bit)

You are also climbing on the mirrors. Your demonstration posted that the gradient in photoshop produces an RGB with non-linear values is wrong. This is a fact. Create an 8 or 16 bit RGB layer. And the screenshoot is obvious is to 8 bit. Is a JPEG on web. But i posted screenshoot that show 16 bit. Creates a gradient fill from "BLACK TO WHITE". Check the values and they will be correct and not as you posted. Do you want see a demonstration video?

anchev wrote:
Your file may be 16bpc but your measurement is 8-bit, which makes no sense in the context of what was previously shown as you are reducing the accuracy 256 times. Obviously you won't be able to notice the small differences with such measurement.

Also it is obvious even for a layman that on your screenshot from sample #1 to sample #2 there are 3 levels and from #2 to #3 there are 4 levels. Even if one assumes that your gradient is "correct":

#1 to #2:
78 - 60 = 18 values
18 / 3 levels = 6 values per level

#2 to #3:
105 - 78 = 27 values
27 / 4 levels = 6.75 per level

6 != 6.75

That proves that your gradient is not linear. Simple maths.

I really don't know what you are trying to do.

Agree the same. I do not understand what you wanted to show with your screenshots and the differences with Gimp. The values you placed did not make sense. And it does not hit the 8-bit measurement with the values shown in the info palette.

fireshoot wrote:
I do not understand what you wanted to show with your screenshots and the differences with Gimp.

Because you don't look.

Gimp:
R = G = B (as it should be).
The gradient is linear.

PS:
R != G = B.
The gradient is not linear.

You quote it, argue about it but you have not read it

you have deliberately chosen gradient levels for which the RGB

do you think i'm playing?.....................
Ok watch this and download my file 1000x1000 with gradient adjustment layer.
or watch my video.
Watch when I scroll the mouse along the gradient, the values are never incorrect but always the same. Are ALWAYS the same.
https://www.dropbox.com/s/tm6p7u62bb1kz2z/vi.mp4?dl=0
Nothing changes even if you set 1000x1000. The values are always the same and you can check it on my file. Otherwise you have some problems with YOUR adobe photoshop.

This is file 1000x1000 with gradient adjustment layer.
https://www.dropbox.com/s/956m936yfexp165/aa.rar?dl=0
and this is with Prophoto Profile. The same results.
https://www.dropbox.com/s/20w9ubsl1zt9r … o.rar?dl=0

anchev wrote:
So as I thought you are doing something different:

1. Your file is 50x40px
2. You did not share a file with gradient adjustment layer.
3. You did not share info about your PS version.

Based on this comparison is not possible.

As for your measurement: It seems to me you have deliberately chosen gradient levels for which the RGB values match (because for these particular ones which you show I also get the same). So this is skewed. You have to measure the same values which I measure.

fireshoot wrote:
my version of PS is Adobe CC 2018

Obviously that's the key.
Good if they have fixed it.

anchev wrote:
Obviously that's the key.
Good if they have fixed it.

Ok, Now we understood where was the problem. In Photoshop version. Unfortunately I do not own the previous one and I have no feedback if not your posts. In the end we say that FINALLY ADOBE FIXED THAT PROBLEM.
Goodnight